Tensioning devices, such as hydraulic tensioners, are used as a control device for a power transmission chain, or any similar power transmission devices, as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises, slippage, or the unmeshing of teeth in cases of a toothed chain. Prevention of such slippage is especially important in the case of a chain driven camshaft in an internal combustion engine because jumping of teeth will throw off the camshaft timing by several degrees, possibly rendering the engine inoperative or causing damage. However, in the harsh environment in which an internal combustion engine chain tension can vary between excessively high or low levels as a result of the wide variations in temperature and differences between the coefficients of linear expansion among the various parts of the engine, including the chain and the tensioner. Moreover, wear to the chain components, during prolonged use, can result in a decrease in the tension of the chain. It is also necessary to provide some measures to remove excessive tensioning forces on the tight side of the chain and to insure the necessary tension forces on the slack side of the chain. In addition, camshaft and crankshaft induced torsional vibrations cause belt tension to vary considerably. This tension variation results in chain elongation.
One example of a device used to control tension in a wrapped power transmission device is described in Biedermann, U.S. Pat. No. 4,713,043. Biedermann discloses a hydraulic ball-type check valve tensioner having a plunger slidably fitted into a chamber and biased by a spring in a protruding direction. The plunger extends against a lever arm that imparts tension to a chain according to the degree of slackening of the chain. A clearance, which is formed between the ball and seat of a check valve, permits the free flow of fluid therethrough into the chamber. Therefore, the hydraulic pressure from an external source, such as an oil pump or the like, flows into the chamber through passages formed in the housing, advancing the plunger easily by the combined efforts of the hydraulic pressure and the spring force.
On the other hand, when the plunger tends to move in a reverse direction, the ball is tightly contacted with the ball seat to restrict outflow of fluid from the chamber. Only a small clearance between the plunger and the housing wall permits some fluid to escape thereby allowing the plunger to retract. In such a fashion, the tensioner achieves a so-called no-return function, i.e., movements are easy in one direction but difficult in the reverse direction.
However, this no-return function may present difficulties in accommodating tension spikes or surges in the chain, belt or similar wrapped power transmission devices. When a timing device operates at its resonant frequency, the chain load increases significantly. The small clearance between the plunger and the housing wall is not sufficient to quickly release the hydraulic fluid in the chamber to accommodate the sudden overload of the chain.
One example of an attempt to alleviate this problem in a hydraulic tensioner is described in Suzuki, U.S. Pat. No. 4,881,927. Suzuki discloses a hydraulic ball-type check valve tensioner having a plunger slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve having a sleeve slidably fitted in an auxiliary chamber in communication with the first chamber, with a spring biasing the sleeve into a depressed position to block a discharge part. Oil in the first chamber flows into the auxiliary chamber to force the sleeve against the biasing spring action to unblock the discharge port. Unfortunately, this relief valve may be slow to open and close due to high mass and subject to variable friction between the sleeve and auxiliary chamber wall. This may vary the pressure at which the relief valve operates.
Another example of an attempt to provide a hydraulic tensioner with a relief valve is described in Mittermeier, U.S. Pat. No. 4,507,103. Mittermeier discloses a hydraulic ball-type check valve tensioner having a plunger slidably fitted into a chamber and biased by a spring in a protruding direction. This tensioner includes a relief valve in a bore at the protruding end of the plunger. This relief valve is a spring-biased ball type valve with the spring set against a threaded throttle plug capping the bore. Oil in the first chamber forces open the ball, upon reaching a set high pressure, and flows into the bore, past the throttle plug threads to the atmosphere. Unfortunately, this relief valve may be slow to release large displacements of oil because of the restricted path past the threads and resultant back-pressure build up against the ball.
Accordingly, it is an object of the present invention to provide a tensioner for chain, belt or similar wrapped power transmission devices which can maintain a substantially constant tensioning force.
It is also an object of the present invention to provide a hydraulic tensioner with a pressure relief valve to allow the plunger to return when excessive loads are seen by the chain.
It is also a further object of the present invention to provide a hydraulic tensioner with a pressure relief valve that has a high spring rate and low mass to give a high frequency response.